1. Field of the Invention
This invention relates generally to the art of transmitting telemetric and control information through a hollow well bore, drill string or other pipe. More particularly, the invention relates to an improved acoustic communication method and system for the purpose described wherein the information carrier is provided by acoustic noise generated by the drill system during drilling operations, preferably torsional waves of zero order inherently generated in the drill string by virtue of drilling operations.
2. Description of the Prior Art
As will appear from the ensuing description, the present acoustic communication method and system may be employed to transmit information, during and by virtue of drilling operations, between two points of a drill pipe having an intervening length capable of sustaining acoustic waves, particularly torsional waves of zero order. However, the principal application of the invention involves transmission of telemetric and control information through a hollow drill string suspended within a well bore. Accordingly, the invention will be disclosed in connection with this particular application.
When drilling a well bore, it is desirable, if not essential, to monitor selected drilling parameters in the vicinity of the drill bit for the purpose of providing the drilling operator with sufficient information to properly control the drilling operation. Among the drilling parameters which provide valuable information to the drilling operation are temperature, formation pressure, formation porosity, and others. See our parent U.S. Pat. No. 3,790,930. In slant drilling operations, such as off-shore drilling of multiple wells from a single platform or island, an additional drilling parameter which provides extremely valuable if not essential information to the drilling operator is drill string orientation.
Such drill string orientation is expressed in terms of the azimuth and pitch or inclination of the lower end of the string and must be accurately measured at frequent intervals during the drilling operation in order to maintain the proper slant drilling direction.
At the present time the most widely used method of measuring drill string orientation involves the use of a well log which is lowered on a cable through the hollow drill string to the bottom of the well bore. This log contains instruments, such as a compass and a spirit level or pendulum, for sensing drill string azimuth and inclination and a camera for photographically recording the instrument readings. After actuation of the camera to record these instrument readings, the log is withdrawn from the drill string and the film is developed to obtain the readings. While this method provides accurate information concerning drill string orientation, it is extremely time-consuming and substantially increases the total drilling cost. Thus, each well logging operation involves cessation of drilling, uncoupling the drilling kelly from the drill string, lowering and subsequently raising the log the whole length of the drill string, recoupling the kelly to the drill string, and resumption of drilling. In many offshore drilling operations, periodic logging of the well bore in this fashion may account for up to one-half the total drilling operation time and hence for a large portion of the total drilling operation cost.
The present invention proposes to avoid the above and other disadvantages of the described well logging technique and to improve on the existing techniques for monitoring other drilling parameters by providing a novel and improved acoustic communication method and system for transmitting telemetric and control information through a drill string during drilling operations, as broadly defined hereinafter.
The prior art relating to well drilling is replete with a vast assortment of acoustic communication techniques for transmitting information through a drill string. Simply stated, such communication techniques involve propagation through the drill string from one position of station to another of modulated acoustic waves containing the information to be transmitted, and demodulation of the modulated waves at another position or station along the drill string to recover the transmitted information. In the present disclosure, the station from which the modulated acoustic waves propagate is referred to as a signal-transmitting station. The position at which the modulated waves are demodulated to recover the transmitted information is referred to as a signal-receiving station.
The prior acoustic communication systems for transmitting information through a drill string are deficient in that they utilize relatively inefficient modes of acoustic wave propagation and thus achieve, at best, only marginal information transmission. In this regard, it is significant to note that most published patents in the field use such descriptors as vibrations, sound, acoustic waves, and the like, to describe the acoustic information carrier, and do not specifically define the exact mode of acoustic wave propagation. For the most part, those patents which do describe a specific form a acoustic wave propagation utilize either longitudinal or flexural vibration modes. These latter vibration modes, however, are ill-suited for use in transmitting information through a drill string of great length owing to the large transmission losses which occur as a result of acoustic coupling of the drill string to the drilling fluid and the wall of the well bore.
Furthermore, this prior manner of operation has required installation of an acoustic wave transducer and its electronic driving circuitry within the lower end of the drill string. Thus, a large source of power to drive the transducer was required at the lower end of the drill string.